Stem cells: a plant biology perspective

A recent meeting at the Juan March Foundation in Madrid, Spain brought together plant biologists to discuss the characteristics of plant stem cells that are unique and those that are shared by stem cells from the animal kingdo

Embryo-patterning genes and reinforcement cues determine cell fate in the Arabidopsis thaliana root

The majority of plant organs arise from groups of continuously dividing cells, the meristems. Little is known about mechanisms of cell specification in meristems. Within the Arabidopsis root meristem, the fate of every cell can be predicted accurately, and the origin of these cells during the formation of the embryonic root primordium is known. Laser ablations reveal that, despite the regularity in cell lineage, position remains important to reinforce cell specification. Genetic analysis has revealed that many genes involved in the specification of the main cell types in the root act early, during embryogenesis, and an important question is whether the same or other genes are involved in the reinforcement of specification. Sub-specification of cell types, as exemplified by epidermal root hair cell specification, involves two pathways, one of which may act to reinforce earlier patterning events mediated by the other

A LEAFY link from outer space

The beautiful shape and colour patterns of flowers attract almost everybody, including developmental biologists. Studies of floral patterning have shown that the identity of floral organs is determined by a combinatorial code of homeotic genes, termed A, B and C, expressed in three separate regions of the flower. This knowledge allows scientists (and biotech companies) to manipulate flowers to form organs at any position, yet the secret of how the three activities are put into the right place remains hidden. But on page 561 of this issue, Parcy et al. bring new insights to this matter. They show that the LEAFY (LFY) protein is directly involved in activating the transcription of homeotic genes, together with at least one co-regulator, UNUSUAL FLORAL ORGANS (UFO). The genes that encode these proteins were originally isolated as floricaula and fimbriata in a flower that almost all of us have manipulated as children, the snapdragon

Cell signaling in root development

Cell signaling has recently been shown to be of major importance in cell specification during Arabidopsis root development. In the ground tissue, cues of unknown molecular nature convey positional information and two genes provide an interesting link between asymmetric cell division and the determination of cell fate. In the root epidermis, cell specification involves ethylene signaling and transcription factors of which at least two are also required for cell fate decisions in the shoot epidermis

Root development: new meanings for root canals?

During Arabidopsis root development, a radial pattern of tissues is extended by the meristem. These tissues form continuous layers and recent data suggest that tissue continuity is instrumental for constraining the direction of signaling in a process termed channeling. In the ground tissue, fate-determining signals originate from contiguous cells of the same layer, possibly due to specific symplastic connections. Mutant analysis supports the hypothesis that vascular tissue continuity may facilitate and depend on the directional transport of a vascular fate-determining signal, possibly the phytohormone auxin

Digging out Roots: Pattern Formation, Cell Division, and Morphogenesis in Plants

The analysis of plant development by genetic, molecular, and surgical approaches has accumulated a large body of data, and yet it remains a challenge to uncover the basic mechanisms that are operating. Early steps of development, when the zygote and its daughter cells organize the embryonic plant, are poorly understood despite considerable efforts toward the identification of relevant genes. Reported cases of genetic redundancy suggest that the difficulty in uncovering patterning genes may reflect overlapping gene activities. Our current knowledge on plant embryo development still leaves open whether mechanisms for axis formation and subsequent pattern formation are fundamentally different in animals and plants. Axis formation may follow the general principle of establishing a peripheral asymmetric cue and mobilizing the cytoskeleton toward this cue-in the case of plants possibly located in the cell wall-but the molecules involved may be entirely different. Embryonic pattern formation involves the establishment of different domains, but although there are candidates, it is not clear whether genes that define these domains are identified yet. Pattern formation continues postembryonically in the meristem, and the flexibility of this process may be explained by a feed-forward system of patterning cues originating from more mature cells. Control of cell division and differentiation, which is important in the meristems-regions of continuous development-has been studied intensively and appears to involve short-range signaling and transmembrane receptor kinase activation. Finally, although high importance of control of cell division rates and planes for plant morphogenesis have been often inferred, recent genetic studies as well as comparative morphological data point to a less decisive role of cell division and to global controls of as yet unknown nature